Method for manufacturing carboxymethylated cellulose nanofiber

ABSTRACT

Provided is a novel manufacturing method whereby a carboxymethylated cellulose nanofiber dispersion having high tarnasparency can be obtained economically. In carboxymethylation of cellulose in the present invention, mercerization is performed in water as the main solvent, after which carboxymethylation is performed in a solvent mixture of water and an organic solvent, By defibrating the resultant carboxymethylated cellulose, a carboxymethylated cellulose nanofiber dispersion having high transparency can be obtained economically.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage filing, under 35 U.S.C. §371(c), of International Application No. PCT/JP2018/044677, filed onDec. 5, 2018, which claims the benefit of priority to Japanese PatentApplication No. 2017-235016, filed on Dec. 7, 2017, and Japanese PatentApplication No. 2018-076540, filed on Apr. 12, 2018. The entire contentsof each of the aforementioned applications are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a novel method for producing acarboxymethylated cellulose nanofiber.

BACKGROUND ART

Carboxymethylated celluloses are cellulose derivatives, and are obtainedby linking carboxymethyl groups to some of hydroxyl groups in glucoseresidues constituting cellulose backbones to form ether linkages.Increase in the amount of carboxymethyl groups (namely, increase in thedegree of carboxymethyl substitution) allows carboxymethylatedcelluloses to be dissolved in water. On the other hand, adjusting thedegree of carboxymethyl substitution to a proper range allowscarboxymethylated celluloses to maintain fibrous shapes in water. Acarboxymethylated cellulose having a fibrous shape can be mechanicallydefibrated and thus converted into a nanofiber having a nanoscale fiberdiameter (PTL 1).

A commonly known method for producing carboxymethylated cellulose is amethod including subjecting cellulose to an alkaline treatment(mercerization) and then a treatment with an etherifying agent (alsoreferred to as “carboxymethylation agent”) (carboxymethylation, alsocalled “etherification”). In the method, there are two known processes,one of which includes performing both mercerization andcarboxymethylation by using water as a solvent and another of whichincludes performing both mercerization and carboxymethylation in anorganic solvent or a mixed solvent of an organic solvent and water (PTL2), and the former is called “water mediated method” and the latter iscalled “solvent mediated method”.

A known method for producing a cellulose nanofiber having a nanoscalefiber diameter is, for example, not only mechanical defibration ofcarboxymethylated cellulose, but also mechanical defibration of acellulose into which carboxyl groups have been introduced (PTL 3). Awater dispersion of a cellulose nanofiber obtained by defibration ofsuch a carboxylated cellulose is known to be high in transparency. Onthe other hand, a water dispersion of a cellulose nanofiber obtained bydefibration of a carboxymethylated cellulose obtained by a solventmediated method is low in transparency as compared with a waterdispersion of a cellulose nanofiber obtained by defibration of acarboxylated cellulose. For obtaining a water dispersion of a cellulosenanofiber with enhanced transparency by defibration of acarboxymethylated cellulose obtained by a water mediated method, a largeamount of chemical agents such as a mercerizing agent and/or acarboxymethylation agent is needed, which has large problems withproduction and economy. Transparent materials are suitable for variousapplications and therefore it is demanded that cellulose nanofibers aretransparent. In particular, carboxymethylated celluloses are materialshigh in safety and thus it is demanded to obtain a cellulose nanofiberhigh in transparency by use of carboxymethylated cellulose in aneconomic manner.

CITATION LIST Patent Literature

-   PTL 1: International Publication No. WO 2014/088072-   PTL 2: Japanese Patent Laid-Open No. 2017-149901-   PTL 3: Japanese Patent Laid-Open No. 2008-1728

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a novel productionmethod which provides a carboxymethylated cellulose nanofiber dispersionhigh in transparency.

Solution to Problem

The present inventors have made intensive studies to achieve the aboveobject, and as a result, have found that when mercerization (alkalinetreatment of cellulose) in a solvent containing mainly water is followedby carboxymethylation (also referred to as “etherification”) in a mixedsolvent of water and an organic solvent in carboxymethylation ofcellulose, a carboxymethylated cellulose which can be used to form acellulose nanofiber dispersion very high in transparency by defibration,can be produced at a high rate of effective utilization of acarboxymethylation agent in an economic manner, as compared with anycarboxymethylated cellulose obtained according to a conventional watermediated method (method including performing both mercerization andcarboxymethylation in water as a solvent) or solvent mediated method(method including performing both mercerization and carboxymethylationin an organic solvent or in a mixed solvent of water and an organicsolvent).

The present invention provides the following, but is not limitedthereto.

(1) A method for producing a carboxymethylated cellulose nanofiber, themethod comprising:

treating cellulose with a mercerizing agent to prepare mercerizedcellulose;

reacting the mercerized cellulose with a carboxymethylation agent toprepare carboxymethylated cellulose; and

defibrating the carboxymethylated cellulose to produce acarboxymethylated cellulose nanofiber; wherein

preparing mercerized cellulose is performed in a solvent containingmainly water, and preparing carboxymethylated cellulose is performed ina mixed solvent of water and an organic solvent.

(2) The method according to (1), wherein the solvent containing mainlywater in preparing mercerized cellulose is a solvent containing higherthan 50% by mass of water.

(3) The method according to (2), wherein the solvent containing mainlywater in preparing mercerized cellulose is water.

(4) The method according to any one of (1) to (3), wherein a rate ofeffective utilization of the carboxymethylation agent is 15% or more.

(5) The method according to any one of (1) to (4), wherein themercerizing agent comprises sodium hydroxide, lithium hydroxide,potassium hydroxide, or a combination of two or more thereof.

(6) The method according to any one of (1) to (5), wherein thecarboxymethylation agent comprises monochloroacetic acid or sodiummonochloroacetate.

(7) The method according to any one of (1) to (6), wherein the mixedsolvent in preparing the carboxymethylated cellulose is a solventcontaining 20 to 99% by mass of an organic solvent.

(8) The method according to any one of (1) to (7), wherein the organicsolvent comprises isopropanol, methanol, ethanol, acetone, or acombination of two or more thereof.

(9) The method according to any one of (1) to (8), wherein a degree ofcarboxymethyl substitution per anhydrous glucose unit in thecarboxymethylated cellulose is less than 0.50.

(10) The method according to any one of (1) to (9), wherein a degree ofcrystallization of cellulose I type in the carboxymethylated celluloseis 50% or more.

Effects of Invention

The method of the present invention can produce a carboxymethylatedcellulose at a high rate of effective utilization of acarboxymethylation agent, the carboxymethylated cellulose forming acellulose nanofiber dispersion very high in transparency by defibration.

DESCRIPTION OF EMBODIMENTS

The present invention relates to a method for producing acarboxymethylated cellulose nanofiber. The carboxymethylated cellulosehas a structure formed by linking carboxymethyl groups to some ofhydroxyl groups in glucose residues constituting cellulose. Thecarboxymethylated cellulose may be in the form of a salt, and examplesof the salt of the carboxymethylated cellulose include metal salts suchas a carboxymethylated cellulose sodium salt.

Carboxymethylated celluloses can be commonly produced by subjectingcellulose to an alkaline treatment (mercerization), and thereafterallowing the resulting mercerized cellulose (also referred to as“alkaline cellulose”) to react with a carboxymethylation agent (alsoreferred to as “etherifying agent”).

<Cellulose>

The cellulose in the present invention means polysaccharide having astructure in which D-glucopyranoses (D-glucopyranose is also simplyreferred to as “glucose residue” or “anhydrous glucose”) are connectedby β-1,4 linkages. Celluloses are commonly classified to, for example,native cellulose, regenerated cellulose, fine cellulose, andmicrocrystalline cellulose from which an amorphous region is removed,depending on the source, the production method, and the like. Any ofsuch celluloses can be used as a raw material of the mercerizedcellulose in the present invention.

Examples of the native cellulose include bleached pulp and unbleachedpulp (bleached wood pulp or unbleached wood pulp); linter and refinedlinter; and cellulose produced by microorganisms such as acetic acidbacteria. The raw material of the bleached pulp or unbleached pulp isnot limited, and examples thereof include wood, cotton, straw, bamboo,hemp, jute, and kenaf. The method for producing the bleached pulp orunbleached pulp is also not particularly limited, and may be amechanical method, a chemical method, or a combined intermediate methodbetween these two methods. Examples of the bleached pulp or unbleachedpulp classified according to the production method include mechanicalpulp (thermomechanical pulp (TMP), groundwood pulp), chemical pulp(sulfite pulp such as needle (softwood) unbleached sulfite pulp (NUSP),and needle bleached sulfite pulp (NBSP), and kraft pulp such as needleunbleached kraft pulp (NUKP), needle bleached kraft pulp (NBKP), leaf(hardwood) unbleached kraft pulp (LUKP), and leaf bleached kraft pulp(LBKP)). Dissolving pulp may also be used, besides papermaking pulp.Dissolving pulp is pulp chemically refined, is mainly used in adissolved state in chemicals, and serves as a main raw material of anartificial fiber, cellophane, or the like.

Examples of the regenerated cellulose include one obtained by dissolvingcellulose in a solvent such as a cuprammonium solution, a cellulosexanthate solution, or a morpholine derivative, and anew subjecting theresultant to spinning. Examples of the fine cellulose include oneobtained by subjecting a cellulose material such as the native celluloseor regenerated cellulose to a depolymerization treatment (for example,acid hydrolysis, alkali hydrolysis, enzymatic degradation, a blastingtreatment, or a vibration ball mill treatment), and one obtained bymechanically treating the cellulose-based material.

<Mercerization>

The mercerized cellulose (also referred to as “alkaline cellulose”) isobtained by using the above cellulose as a raw material and adding amercerizing agent (alkali) thereto. In the present invention, acarboxymethylated cellulose which can be defibrated to form a cellulosenanofiber dispersion very high in transparency can be provided in aneconomic manner, by use of water as a solvent in a mercerizationreaction and a mixed solvent of an organic solvent and water in the nextcarboxymethylation.

“Using mainly water as a solvent (solvent containing mainly water)”refers to using a solvent containing water in an amount of higher than50% by mass. The solvent containing mainly water preferably contains 55%by mass or more, more preferably 60% by mass or more, more preferably70% by mass or more, more preferably 80% by mass or more, furtherpreferably 90% by mass or more, further preferably 95% by mass or moreof water. The solvent containing mainly water particularly preferablycontains 100% by mass of water (namely, is water). As the proportion ofwater in mercerization is higher, the transparency of a cellulosenanofiber dispersion obtained by defibrating the carboxymethylatedcellulose is higher. Examples of the solvent (which is mixed with waterwhen use) other than water in the solvent containing mainly waterinclude an organic solvent used as a solvent in carboxymethylation ofthe later stage. Examples can include alcohols such as methanol,ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, andtert-butanol, ketones such as acetone, diethyl ketone, and methyl ethylketone, and dioxane, diethyl ether, benzene and dichloromethane. Thesecan be used singly or in the form of a mixture of two or more thereof,and be added to water in an amount of less than 50% by mass, as asolvent for mercerization. The solvent containing mainly waterpreferably contains 45% by mass or less, further preferably 40% by massor less, further preferably 30% by mass or less, further preferably 20%by mass or less, further preferably 10% by mass or less, furtherpreferably 5% by mass or less, more preferably 0% by mass of the organicsolvent.

Examples of the mercerizing agent include alkali metal hydroxides suchas lithium hydroxide, sodium hydroxide, and potassium hydroxide, andthese can be used singly or in combinations of any two or more thereof.The mercerizing agent is not limited thereto, but such an alkali metalhydroxide can be added to a reactor, for example, in the form of anaqueous solution having a content of the alkali metal hydroxide of 1 to60% by mass, preferably 2 to 45% by mass, more preferably 3 to 25% bymass. The amount of the mercerizing agent used is preferably 0.1 mol ormore and 2.5 mol or less, more preferably 0.3 mol or more and 2.0 mol orless, further preferably 0.4 mol or more and 1.5 mol or less, per 100 gof cellulose (absolute dry), in one embodiment.

The amount of the solvent containing mainly water in mercerization maybe any amount as long as a raw material can be stirred and mixed, and isnot particularly limited, and the amount thereof is preferably as largeas that of cellulose as a raw material, in terms of mass.

A mercerization treatment is performed by mixing a starting raw material(cellulose) and the solvent containing mainly water, adjusting thetemperature of a reactor to 0 to 70° C., preferably 10 to 60° C., morepreferably 10 to 40° C., adding an aqueous solution of a mercerizingagent, and stirring the resultant for 15 minutes to 8 hours, preferably30 minutes to 7 hours, more preferably 30 minutes to 3 hours. Thus, themercerized cellulose (alkaline cellulose) is obtained.

The pH in mercerization is preferably 9 or more, and thus themercerization reaction can progress. The pH is more preferably 11 ormore, further preferably 12 or more, and may be 13 or more. The upperlimit of the pH is not particularly limited.

Such mercerization can be performed by use of a reactor in which theabove respective components can be mixed and stirred while controllingthe temperature, and any of various reactor conventionally used in amercerization reaction can be used. For example, a batch type stirredreactor in which two screws are used to stir and mix the components ispreferable in view of uniform mixing ability and also productivity.

<Carboxymethylation>

The carboxymethylated cellulose is obtained by adding acarboxymethylation agent (also referred to as “etherifying agent”) tothe mercerized cellulose. In the present invention, a mixed solvent ofwater and an organic solvent is used as a solvent in acarboxymethylation reaction. A carboxymethylated cellulose which can bedefibrated to form a cellulose nanofiber dispersion very high intransparency can be obtained in an economic manner, by use of a solventcontaining mainly water in mercerization and a mixed solvent of waterand an organic solvent in carboxymethylation.

Examples of the carboxymethylation agent include monochloroacetic acid,sodium monochloroacetate, methyl monochloroacetate, ethylmonochloroacetate, and isopropyl monochloroacetate. In particular,monochloroacetic acid or sodium monochloroacetate is preferable in termsof their availability. The carboxymethylation agent is preferably addedin an amount ranging from 0.5 to 1.5 mol per anhydrous glucose unit ofcellulose. The lower limit of the range is more preferably 0.6 mol ormore, further preferably 0.7 mol or more, and the upper limit in therange is more preferably 1.3 mol or less, further preferably 1.1 mol orless. The carboxymethylation agent can be added to a reactor, forexample, but not limited to, in the form of an aqueous solution having acontent of the carboxymethylation agent of 5 to 80% by mass, morepreferably 30 to 60% by mass, and can also be added in the form of apowder without any dissolution.

The molar ratio of the mercerizing agent to the carboxymethylation agent(mercerizing agent/carboxymethylation agent) generally adopted is 0.90to 2.45 in a case where monochloroacetic acid or sodiummonochloroacetate is used as the carboxymethylation agent. The reasonfor this is as follows: a molar ratio of less than 0.90 can cause acarboxymethylation reaction to insufficiently progress, resulting inremaining of the unreacted monochloroacetic acid or sodiummonochloroacetate and thus diseconomy, and a molar ratio of more than2.45 may cause a side reaction of an excess of the mercerizing agentwith monochloroacetic acid or sodium monochloroacetate to progress,resulting in production of a glycolic acid alkali metal salt and thusdiseconomy.

In the present invention, the rate of effective utilization of thecarboxymethylation agent is preferably 15% or more, more preferably 20%or more, further preferably 25% or more, particularly preferably 30% ormore. The rate of effective utilization of the carboxymethylation agentrefers to the proportion of carboxymethyl groups introduced intocellulose to carboxymethyl groups in the carboxymethylation agent. Thepresent invention can provide a carboxymethylated cellulose which can bedefibrated to form a cellulose nanofiber dispersion high intransparency, at a high rate of effective utilization of thecarboxymethylation agent (namely, in an economic manner without use oflarge amount of the carboxymethylation agent), by use of a solventcontaining mainly water in mercerization and a mixed solvent of waterand an organic solvent in carboxymethylation. The upper limit of therate of effective utilization of the carboxymethylation agent is notparticularly limited, and the upper limit is actually about 80%. Therate of effective utilization of the carboxymethylation agent may beherein abbreviated as AM.

The method for calculating the rate of effective utilization of thecarboxymethylation agent is as follows:AM=(DS×Number of moles of cellulose)/Number of moles ofcarboxymethylation agentDS: Degree of carboxymethyl substitution (the measurement method will bedescribed below)Number of moles of cellulose: Mass of pulp (Dry mass after drying at100° C. for 60 minutes)/162(162 means the molecular weight per glucose unit of cellulose).

The concentration of the raw material of cellulose in thecarboxymethylation reaction is not particularly limited, and ispreferably 1 to 40% (w/v) in view of an enhancement in rate of effectiveutilization of the carboxymethylation agent.

A mixed solvent of water and an organic solvent is formed byappropriately adding an organic solvent or an aqueous solution of anorganic solvent to the reactor, or alternatively, appropriately reducingthe organic solvent other than water for the mercerization treatment,for example, under reduced pressure, at the same time as of addition ofthe carboxymethylation agent or before or immediately after addition ofthe carboxymethylation agent. The carboxymethylation reaction is allowedto progress in the mixed solvent of water and an organic solvent in thepresent invention. The timing of addition or reduction of the organicsolvent is not particularly limited as long as it is within the timefrom completion of the mercerization reaction to immediately afteraddition of the carboxymethylation agent, and is preferably, forexample, within 30 minutes before or after addition of thecarboxymethylation agent.

Examples of the organic solvent include alcohols such as methanol,ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, andtert-butanol, ketones such as acetone, diethyl ketone, and methyl ethylketone, and dioxane, diethyl ether, benzene and dichloromethane. Thesecan be used singly or in the form of a mixture of two or more thereof,and be added to water as a solvent for carboxymethylation. Inparticular, a monohydric alcohol having 1 to 4 carbon atoms ispreferable, and a monohydric alcohol having 1 to 3 carbon atoms isfurther preferable, because compatibility with water is excellent.

The content of the organic solvent in the mixed solvent forcarboxymethylation is preferably 20 to 99% by mass, more preferably 30to 99% by mass, further preferably 40 to 99% by mass, further preferably45 to 99% by mass based on the total of water and the organic solvent.

The reaction medium (the mixed solvent of water and the organic solvent,containing no cellulose) for carboxymethylation preferably has a lowerwater content (in other words, a higher organic solvent content) thanthe reaction medium for mercerization. When the above range issatisfied, the degree of crystallization of the resultingcarboxymethylated cellulose can be easily maintained while the degree ofcarboxymethyl substitution is also easily increased, and thus acarboxymethylated cellulose which can be defibrated to form a cellulosenanofiber dispersion high in transparency can be more efficientlyobtained. In a case where the reaction medium for carboxymethylation hasa lower water content (a higher organic solvent content) than thereaction medium for mercerization, the mixed solvent for thecarboxymethylation reaction can be advantageously formed by a simpleprocedure in which a desired amount of the organic solvent is added tothe reaction system after completion of the mercerization reaction, intransferring from the mercerization reaction to the carboxymethylationreaction.

The mixed solvent of water and an organic solvent is thus formed, thecarboxymethylation agent is added to the mercerized cellulose, and thenthe resultant is stirred for about 15 minutes to 4 hours, preferablyabout 15 minutes to 1 hour while the temperature is preferably keptconstantly in the range from 10 to 40° C. Mixing of a liquid containingthe mercerized cellulose with the carboxymethylation agent is preferablyperformed by adding the carboxymethylation agent in portions ordropwise, in view of preventing the reaction mixture from being at ahigh temperature. After the carboxymethylation agent is added, theresulting mixture was stirred for a certain time, and the temperaturethereof is, if necessary, raised. An etherification (carboxymethylation)reaction is then performed at a reaction temperature of 30 to 90° C.,preferably 40 to 90° C., further preferably 60 to 80° C. for 30 minutesto 10 hours, preferably 1 hour to 4 hours, thereby obtaining thecarboxymethylated cellulose.

The reactor which has been used in mercerization may be used as it is,or another reactor in which the components can be mixed and stirredwhile controlling the temperature may be used, in carboxymethylation.

After completion of the reaction, the remaining alkali metal salt may beneutralized with a mineral acid or organic acid. If necessary, aninorganic salt, an organic acid salt, and others as by-product(s) may beremoved by washing with water-containing methanol, and the resultant maybe dried, pulverized and classified to form the carboxymethylatedcellulose or the salt thereof. Examples of an apparatus for use in drypulverization include impact mills such as a hammer mill and a pin mill,medium mills such as a ball mill and a tower mill, and jet mills.Examples of an apparatus for use in wet pulverization includeapparatuses such as a homogenizer, a masscolloider, and a pearl mill.

<Carboxymethylated Cellulose>

A carboxymethylated cellulose produced according to the presentinvention preferably maintains at least a portion of a fibrous shape,when dispersed in water. Specifically, it is preferable that a fibroussubstance is found in observation of a water dispersion of thecarboxymethylated cellulose with an electron microscope, and that a peakof a cellulose I type crystal is exhibited in observation of thecarboxymethylated cellulose with X-ray diffraction.

The carboxymethylated cellulose maintaining at least a portion of afibrous shape, when dispersed in water, has a degree of carboxymethylsubstitution per anhydrous glucose unit of cellulose of less than 0.50.In a case where the degree of substitution is 0.50 or more, dissolutionin water easily occurs to fail to maintain a fibrous form. The degree ofsubstitution is particularly preferably 0.02 or more and less than 0.50,further preferably 0.05 or more and less than 0.50, further preferably0.10 or more and 0.40 or less, further preferably 0.20 or more and 0.40or less in consideration of operationability. By introducingcarboxymethyl groups into celluloses, electrical repulsion of thecelluloses occurs, and thus the resulting celluloses can be defibratedto nanofibers; however, when the degree of carboxymethyl substitutionper anhydrous glucose unit is less than 0.02, defibration intonanofibers may be insufficiently. The degree of carboxymethylsubstitution can be adjusted by controlling the amount of acarboxymethylation agent for reaction to be added, the amount of amercerizing agent, and the compositional ratio between water and anorganic solvent.

The anhydrous glucose unit in the present invention means individualanhydrous glucose (glucose residue) constituting cellulose. The degreeof carboxymethyl substitution (also referred to as “degree ofetherification”) herein represents the proportion of hydroxyl groupsreplaced with carboxymethyl ether groups among hydroxyl groups of theglucose residue constituting cellulose (the number of carboxymethylether groups per glucose residue). The degree of carboxymethylsubstitution may be abbreviated as DS.

The method for measuring the degree of carboxymethyl substitution is asfollows: About 2.0 g of a sample is precisely weighed and is put in a300-mL stoppered conical flask. 100 mL of a liquid obtained by adding100 mL of nitric acid (special grade) to 1000 mL of nitric acid/methanolis added thereto and the resulting flask was shaken for 3 hours, therebyconverting the salt of the carboxymethylated cellulose (CMC) into H-CMC(hydrogen-type carboxymethylated cellulose). 1.5 to 2.0 g of theabsolute dry H-CMC is precisely weighed, and put in a 300-mL stopperedconical flask. The H-CMC is wetted with 15 mL of 80% methanol. 100 mL of0.1 N—NaOH is added thereto, and the resulting flask was shaken at roomtemperature for 3 hours. Phenolphthalein is used as an indicator toreversely titrate excess NaOH by 0.1 N—H₂SO₄, and the degree ofcarboxymethyl substitution (DS value) is calculated according to thefollowing expressions.A=[(100×F′−0.1 N—H₂SO₄(mL)×F)×0.1]/(Absolute dry mass (g) of H-CMC)Degree of carboxymethyl substitution=0.162×A/(1−0.058×A)F′: factor of 0.1 N—HSO₄F: factor of 0.1 N—NaOH.

The degree of crystallization of cellulose in the carboxymethylatedcellulose fiber of the present invention is preferably 50% or more, morepreferably 60% or more, with respect to crystal I type. When thecrystallinity is within the above range, transparency of a cellulosenanofiber dispersion obtained by defibration is enhanced. Thecrystallinity of cellulose can be controlled by the concentration of amercerizing agent and the temperature in treatment, as well as thedegree of carboxymethylation. An alkali at a high concentration is usedin mercerization and carboxymethylation to thereby allow a I typecrystal of cellulose to be easily converted into a II type crystal;however, for example, the amount of the alkali (mercerizing agent) usedcan be adjusted to adjust the degree of denaturation, thereby allowingdesired crystallinity to be maintained.

The method for measuring the degree of crystallization of cellulose Itype of the carboxymethylated cellulose is as follows:

A sample is placed on a glass cell, and subjected to measurement with anX-ray diffractometer (LabX XRD-6000, manufactured by ShimadzuCorporation). The degree of crystallization is calculated according to aprocedure of Segal, et al., and is calculated from the diffractionintensity of the 002 plane at 2θ=22.6° and the diffraction intensity ofan amorphous portion at 2θ=18.5° with the diffraction intensity at2θ=10° to 30° as the baseline in an X-ray diffraction diagram, accordingto the following expressions.Xc=(I002c−Ia)/I002c×100

Xc=degree (%) of crystallization of cellulose I type

I002c: diffraction intensity of 002 plane at 2θ=22.6°

Ia: diffraction intensity of amorphous portion at 2θ=18.5°.

A carboxymethylated cellulose produced by the present invention can beused in the state of a dispersion obtained after the reaction, or canbe, if necessary, dried, and re-dispersed in water before use. Thedrying method is not limited at all, and for example, any known methodsuch as a freeze-drying method, a spray-drying method, a shelf-typedrying method, a drum drying method, a belt drying method, a dryingmethod including thinly extending on a glass plate or the like, afluid-bed drying method, a microwave drying method, or a drying methodincluding using heat generating fan under reduced pressure can be used.After drying, the resultant may be, if necessary, pulverized by a cuttermill, a hammer mill, a pin mill, a jet mill, or the like. The method forre-dispersing in water is also not particularly limited, and any knowndispersing apparatus can be used.

<Production of Carboxymethylated Cellulose Nanofiber>

A carboxymethylated cellulose obtained by the method of the presentinvention can be converted by defibration into a cellulose nanofiberhaving a nanoscale fiber diameter. A carboxymethylated cellulosenanofiber obtained by the method of the present invention can beproduced in an economic manner and has a high transparency in the stateof a water dispersion, as compared with a carboxymethylated cellulosenanofiber obtained by a conventional water mediated method or solventmediated method.

Before defibration, a dispersion of the carboxymethylated celluloseobtained by the method is prepared. A dispersing medium is preferablywater in terms of ease of handling. The concentration of thecarboxymethylated cellulose is preferably 0.01 to 10% (w/v) inconsideration of defibration and dispersing efficiencies.

The apparatus for use in defibration of the carboxymethylated celluloseis not particularly limited, and, for example, a high-speed rotationtype, colloide mill type, high pressure type, roll mill type, orultrasound type apparatus can be used. It is preferable to apply astrong shear force to the dispersion of the carboxymethylated cellulosein defibration. Particularly, it is preferable for efficient defibrationto use a wet high-pressure or ultra-high-pressure homogenizer which canapply a pressure of 50 MPa or more to the dispersion and can apply astrong shear force. The pressure is more preferably 100 MPa or more,further preferably 140 MPa or more. The dispersion may be subjected to,if necessary, a pre-treatment with known mixing, stirring, emulsifying,and/or dispersing apparatus(es), such as a high-speed shear mixer, priorto a defibration and dispersing treatment by a high-pressurehomogenizer.

The high-pressure homogenizer is an apparatus which performsemulsifying, dispersing, defibrating, pulverizing, and ultra-fining withtotal energy of particle collision, a shear force due to the differencein pressure, and the like by subjecting a fluid to pressurizing (highpressure) by a pump and thus ejecting the fluid through a very fine gapprovided in a passage.

Defibration of the carboxymethylated cellulose can provide acarboxymethylated cellulose nanofiber having an average fiber diameterof 3 to 500 nm and an aspect ratio of 50 or more. The average fiberdiameter is preferably 3 to 150 nm, further preferably 3 to 20 nm,further preferably 5 to 19 nm, further preferably 5 to 15 nm.

The average fiber diameter and the average fiber length of thecarboxymethylated cellulose or the carboxymethylated cellulose nanofibercan be each determined by analyzing 200 fibers randomly selected, withan atomic force microscope (AFM) in the case of a diameter of 20 nm orless or with a field emission scanning electron microscope (FE-SEM) inthe case of a diameter of 20 nm or more, and calculating the average.The aspect ratio can be calculated according to the followingexpression:Aspect ratio=Average fiber length/Average fiber diameter.

The degree of carboxymethyl substitution of the carboxymethylatedcellulose nanofiber is generally the same as the degree of carboxymethylsubstitution of the carboxymethylated cellulose before formation of thenanofiber. The proportion of the I type crystal of the carboxymethylatedcellulose nanofiber is generally the same as that of thecarboxymethylated cellulose before formation of the nanofiber.

The method of the present invention can provide a cellulose nanofiberdispersion of the carboxymethylated cellulose with high transparency.The transparency (transmittance of light at 660 nm) of a waterdispersion having a solid content of, for example, 1% (w/v), of acarboxymethylated cellulose nanofiber obtained according to the presentinvention is 50% or more, more preferably 60% or more, furtherpreferably 70% or more, further preferably 80% or more, furtherpreferably 90% or more. Such a cellulose nanofiber can be optimally usedin an application which requires transparency. The present invention canproduce such a cellulose nanofiber high in transparency, at a high rateof effective utilization of the carboxymethylation agent (namely, in aneconomic manner without significant increase in amount of thecarboxymethylation agent).

While the reason why the production method of the present invention canprovide a cellulose nanofiber high in transparency in an economic manneris not clear, the present inventors has confirmed that the productionmethod of the present invention enables maintaining a relatively highdegree of crystallization of cellulose I type and therefore enablesmaintaining a fibrous shape of carboxymethylated cellulose even at arelatively high degree of carboxymethyl substitution. It is consideredthat a high degree of carboxymethyl substitution (namely, introductionof many carboxylmethyl groups) with the fibrous shape being maintainedleads to an enhancement in defibrating properties of carboxymethylatedcellulose, and this is probably one reason why a nanofiber dispersionhigh in transparency is obtained. However, reasons other than the aboveare not excluded.

EXAMPLES

Hereinafter, the present invention will be more specifically describedwith reference to Examples and Comparative Examples, but the presentinvention is not intended to be limited thereto. Unless particularlynoted, “part(s)” and “%” represent “part(s) by mass” and “% by mass”.

Example 1

To a twin-screw kneader whose rotational speed was modulated to 100 rpm,were added 130 parts of water and a solution of 20 parts of sodiumhydroxide in 100 parts of water, and hardwood pulp (LBKP manufactured byNippon Paper Industries Co., Ltd.) was added thereto in an amount of 100parts in terms dry mass when dried at 100° C. for 60 minutes. Theresultant was stirred and mixed at 30° C. for 90 minutes, to preparemercerized cellulose. Furthermore, 230 parts of isopropanol (IPA) and 60parts of sodium monochloroacetate were added thereto while stirring. Theresultant was stirred for 30 minutes, and the temperature was raised to70° C. to allow a carboxymethylation reaction to occur for 90 minutes.The concentration of IPA in a reaction medium in the carboxymethylationreaction was 50%. After completion of the reaction, the resultant wassubjected to neutralization, liquid removal, drying, and pulverization,thereby obtaining a carboxymethylated cellulose sodium salt which had adegree of carboxymethyl substitution of 0.31 and a degree ofcrystallization of cellulose I type of 67%. The rate of effectiveutilization of the carboxymethylation agent was 37%. The methods formeasuring the degree of carboxymethyl substitution and the degree ofcrystallization of cellulose I type, and the method for calculating therate of effective utilization of the carboxymethylation agent are asdescribed above.

The resulting carboxymethylated cellulose sodium salt was dispersed inwater to form a 1% (w/v) water dispersion. The water dispersion wastreated by a high-pressure homogenizer at 150 MPa three times, therebyobtaining a carboxymethylated cellulose nanofiber dispersion. Thetransparency and the viscosity of the resulting dispersion were measuredaccording to the following methods.

<Measurement of Transparency of Cellulose Nanofiber Dispersion>

The transparency (transmittance of light at 660 nm) of the cellulosenanofiber dispersion (solid content: 1% (w/v), dispersing medium: water)was measured with a UV-VIS spectrophotometer UV-1800 (manufactured byShimadzu Corporation).

<Measurement of Viscosity>

The cellulose nanofiber dispersion (solid content: 1% (w/v), dispersingmedium: water) was left to still stand at 25° C. for 16 hours, andthereafter stirred using a stirring machine at 3000 rpm for 1 minute,and the viscosity was measured with a No. 4 rotor at a rotational speedof 60 rpm or 6 rpm after 3 minutes by use of a B-type viscometer(manufactured by Toki Sangyo Co., Ltd).

Example 2

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the amount of IPA added was changedto thereby change the concentration of IPA in the reaction liquid in thecarboxymethylation reaction to 90%. The degree of carboxymethylsubstitution was 0.47, the degree of crystallization of cellulose I typewas 63%, and the rate of effective utilization of the carboxymethylationagent was 56%.

The resulting carboxymethylated cellulose sodium salt was defibrated inthe same manner as in Example 1, thereby obtaining a carboxymethylatedcellulose nanofiber dispersion.

Example 3

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the solvent in the mercerizationreaction was of 90% of water and 10% of IPA and the amount of IPA addedwas changed to thereby allow the concentration of IPA in the mixedsolvent in the carboxymethyl reaction to be adjusted to 50% as inExample 1. The degree of carboxymethyl substitution was 0.28, the degreeof crystallization of cellulose I type was 69%, and the rate ofeffective utilization of the carboxymethylation agent was 34%. Theresulting carboxymethylated cellulose sodium salt was defibrated in thesame manner as in Example 1, thereby obtaining a carboxymethylatedcellulose nanofiber dispersion.

Example 4

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1, except that the solvent for the mercerizationreaction contained 70% of water and 30% of IPA and the amount of IPAadded was changed to thereby allow the concentration of IPA in the mixedsolvent in the carboxymethyl reaction to be adjusted to 50% as inExample 1. The degree of carboxymethyl substitution was 0.28, the degreeof crystallization of cellulose I type was 64%, and the rate ofeffective utilization of the carboxymethylation agent was 34%. Theresulting carboxymethylated cellulose sodium salt was defibrated in thesame manner as in Example 1, thereby obtaining a carboxymethylatedcellulose nanofiber dispersion.

Example 5

A carboxymethylated cellulose lithium salt was obtained in the samemanner as in Example 1 except that lithium hydroxide was used as themercerizing agent instead of sodium hydroxide. The degree ofcarboxymethyl substitution was 0.25, the degree of crystallization ofcellulose I type was 62%, and the rate of effective utilization of thecarboxymethylation agent was 30%. The resulting carboxymethylatedcellulose lithium salt was defibrated in the same manner as in Example1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion.

Example 6

A carboxymethylated cellulose potassium salt was obtained in the samemanner as in Example 1 except that potassium hydroxide was used as themercerizing agent instead of sodium hydroxide. The degree ofcarboxymethyl substitution was 0.25, the degree of crystallization ofcellulose I type was 61%, and the rate of effective utilization of thecarboxymethylation agent was 30%. The resulting carboxymethylatedcellulose potassium salt was defibrated in the same manner as in Example1, thereby obtaining a carboxymethylated cellulose nanofiber dispersion.

Example 7

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the organic solvent added in thecarboxymethylation reaction was changed from IPA to methanol. The degreeof carboxymethyl substitution was 0.29, the degree of crystallization ofcellulose I type was 66%, and the rate of effective utilization of thecarboxymethylation agent was 35%. The resulting carboxymethylatedcellulose sodium salt was defibrated in the same manner as in Example 1,thereby obtaining a carboxymethylated cellulose nanofiber dispersion.

Example 8

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the organic solvent added in thecarboxymethylation reaction was changed from IPA to ethanol. The degreeof carboxymethyl substitution was 0.30, the degree of crystallization ofcellulose I type was 67%, and the rate of effective utilization of thecarboxymethylation agent was 36%. The resulting carboxymethylatedcellulose sodium salt was defibrated in the same manner as in Example 1,thereby obtaining a carboxymethylated cellulose nanofiber dispersion.

Example 9

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the organic solvent added in thecarboxymethylation reaction was changed from IPA to acetone. The degreeof carboxymethyl substitution was 0.26, the degree of crystallization ofcellulose I type was 63%, and the rate of effective utilization of thecarboxymethylation agent was 31%. The resulting carboxymethylatedcellulose sodium salt was defibrated in the same manner as in Example 1,thereby obtaining a carboxymethylated cellulose nanofiber dispersion.

Example 10

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that a solution with 40 parts of sodiumhydroxide in 100 parts of water was used instead of the solution with 20parts of sodium hydroxide in 100 parts of water in the mercerizationreaction and 50 parts of monochloroacetic acid was used instead of 60parts of sodium monochloroacetate as the carboxymethylation agent in thecarboxymethylation reaction. The degree of carboxymethyl substitutionwas 0.31, the degree of crystallization of cellulose I type was 60%, andthe rate of effective utilization of the carboxymethylation agent was36%. The resulting carboxymethylated cellulose sodium salt wasdefibrated in the same manner as in Example 1, thereby obtaining acarboxymethylated cellulose nanofiber dispersion.

Example 11

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the amount of IPA added was changedto thereby change the concentration of IPA in the reaction liquid in thecarboxymethylation reaction to 30%. The degree of carboxymethylsubstitution was 0.24, the degree of crystallization of cellulose I typewas 73%, and the rate of effective utilization of the carboxymethylationagent was 29%. The resulting carboxymethylated cellulose sodium salt wasdefibrated in the same manner as in Example 1, thereby obtaining acarboxymethylated cellulose nanofiber dispersion.

Example 12

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the amount of IPA added was changedto thereby change the concentration of IPA in the reaction liquid in thecarboxymethylation reaction to 2θ%. The degree of carboxymethylsubstitution was 0.20, the degree of crystallization of cellulose I typewas 74%, and the rate of effective utilization of the carboxymethylationagent was 24%. The resulting carboxymethylated cellulose sodium salt wasdefibrated in the same manner as in Example 1, thereby obtaining acarboxymethylated cellulose nanofiber dispersion.

Comparative Example 1

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the solvent for thecarboxymethylation reaction was of 100% of water. The degree ofcarboxymethyl substitution was 0.11, the degree of crystallization ofcellulose I type was 72%, and the rate of effective utilization of thecarboxymethylation agent was 13%. The resulting carboxymethylatedcellulose sodium salt was defibrated in the same manner as in Example 1,thereby obtaining a carboxymethylated cellulose nanofiber dispersion.

Comparative Example 2

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Example 1 except that the solvent for the mercerizationreaction contained 10% of water and 90% of IPA and, and that a solventhaving the same composition was also used in the carboxymethylationreaction. The degree of carboxymethyl substitution was 0.27, the degreeof crystallization of cellulose I type was 64%, and the rate ofeffective utilization of the carboxymethylation agent was 32%. Theresulting carboxymethylated cellulose sodium salt was defibrated in thesame manner as in Example 1, thereby obtaining a carboxymethylatedcellulose nanofiber dispersion.

Comparative Example 3

A carboxymethylated cellulose sodium salt was obtained in the samemanner as in Comparative Example 1 except that a solution of 45 parts ofsodium hydroxide in 100 parts of water was used instead of the solutionof 20 parts of sodium hydroxide in 100 parts of water in themercerization reaction and that 150 parts of sodium monochloroacetatewas used instead of 60 parts of sodium monochloroacetate as thecarboxymethylation agent in the carboxymethylation reaction. The degreeof carboxymethyl substitution was 0.28, the degree of crystallization ofcellulose I type was 45%, and the rate of effective utilization of thecarboxymethylation agent was 13%. The resulting carboxymethylatedcellulose sodium salt was defibrated in the same manner as in Example 1,thereby obtaining a carboxymethylated cellulose nanofiber dispersion.

TABLE 1 Carboxymethylated cellulose Mercerization CarboxymethylationSolvent Solvent Degree of Mercerizing Organic Carboxymethylation Organiccarboxymethyl agent Water solvent agent Water solvent substitutionExample 1 NaOH 100% — Na monochloroacetate 50% IPA50% 0.31 Example 2NaOH 100% — Na monochloroacetate 10% IPA90% 0.47 Example 3 NaOH  90%IPA10% Na monochloroacetate 50% IPA50% 0.28 Example 4 NaOH  70% IPA30%Na monochloroacetate 50% IPA50% 0.28 Example 5 LiOH 100% — Namonochloroacetate 50% IPA50% 0.25 Example 6 KOH 100% — Namonochloroacetate 50% IPA50% 0.25 Example 7 NaOH 100% — Namonochloroacetate 50% Methanol 50% 0.29 Example 8 NaOH 100% — Namonochloroacetate 50% Ethanol 50% 0.30 Example 9 NaOH 100% — Namonochloroacetate 50% Acetone 50% 0.26 Example 10 NaOH 100% —Monochloroacetic acid 50% IPA50% 0.31 Example 11 NaOH 100% — Namonochloroacetate 70% IPA30% 0.24 Example 12 NaOH 100% — Namonochloroacetate 80% IPA20% 0.20 Comparative NaOH 100% — Namonochloroacetate 100%  — 0.11 Example 1 Comparative NaOH  10% IPA90% Namonochloroacetate 10% IPA90% 0.27 Example 2 Comparative NaOH 100% — Namonochloroacetate 100%  — 0.28 Example 3 Carboxymethylated celluloseDegree of Rate of effective crystallization utilization of Cellulosenanofiber dispersion of cellulose carboxymethylation Viscosity ViscosityI type agent (60 pm, mPa · s) (6 rpm, mPa · s) Transparency Example 167% 37% 3700 22000 94% Example 2 63% 56% 2800 20500 92% Example 3 69%34% N.D. N.D. 93% Example 4 64% 34% N.D. N.D. 86% Example 5 62% 30% N.D.N.D. 82% Example 6 61% 30% N.D. N.D. 80% Example 7 66% 35% N.D. N.D. 92%Example 8 67% 36% N.D. N.D. 91% Example 9 63% 31% N.D. N.D. 90% Example10 60% 36% N.D. N.D. 90% Example 11 73% 29% 5350 33000 84% Example 1274% 24% N.D. N.D. 60% Comparative 72% 13% 2350 14000  7% Example 1Comparative 64% 32% N.D. N.D. 10% Example 2 Comparative 45% 13% N.D.N.D. 90% Example 3

It was found from the results in Table 1 that in Examples 1 to 12, inwhich mercerization and carboxymethylation were performed in the solventcontaining mainly water and the mixed solvent of water and an organicsolvent respectively, a cellulose nanofiber dispersion having a veryhigh transparency was produced, as compared with Comparative Example 2(solvent mediated method), in which both mercerization andcarboxymethylation were performed in the solvent containing mainly anorganic solvent, as a conventional method. It was also found that inExamples 1 to 12, a cellulose nanofiber dispersion having a hightransparency was produced with a high rate of effective utilization ofthe carboxymethylation agent, as compared with Comparative Examples 1and 3, in which both mercerization and carboxymethylation were performedwith water as a solvent.

The invention claimed is:
 1. A method for producing a carboxymethylatedcellulose nanofiber, the method comprising: treating cellulose with amercerizing agent to prepare mercerized cellulose; reacting themercerized cellulose with a carboxymethylation agent to preparecarboxymethylated cellulose, wherein the carboxymethylated cellulose isone that maintains at least a portion of a fibrous shape when dispersedin water; and defibrating the carboxymethylated cellulose to produce acarboxymethylated cellulose nanofiber; wherein preparing mercerizedcellulose is performed in a solvent containing 70% by mass or more ofwater, and preparing carboxymethylated cellulose is performed in a mixedsolvent of water and an organic solvent; wherein the mixed solvent inpreparing the carboxymethylated cellulose is a solvent comprising 50 to99% by mass of an organic solvent.
 2. The method according to claim 1,wherein the solvent containing 70% by mass or more of water in preparingmercerized cellulose is water.
 3. The method according to claim 2,wherein a rate of effective utilization of the carboxymethylation agentis 15% or more.
 4. The method according to claim 3, wherein themercerizing agent comprises sodium hydroxide, lithium hydroxide,potassium hydroxide, or a combination of two or more thereof.
 5. Themethod according to claim 4, wherein the carboxymethylation agentcomprises monochloroacetic acid or sodium monochloroacetate.
 6. Themethod according to claim 5, wherein the organic solvent comprisesisopropanol, methanol, ethanol, acetone, or a combination of two or morethereof.
 7. The method according to claim 6, wherein a degree ofcarboxymethyl substitution per anhydrous glucose unit in thecarboxymethylated cellulose is less than 0.50.
 8. The method accordingto claim 7, wherein a degree of crystallization of cellulose I type inthe carboxymethylated cellulose is 50% or more.
 9. The method accordingto claim 1, wherein a rate of effective utilization of thecarboxymethylation agent is 15% or more.
 10. The method according toclaim 1, wherein the mercerizing agent comprises sodium hydroxide,lithium hydroxide, potassium hydroxide, or a combination of two or morethereof.
 11. The method according to claim 1, wherein thecarboxymethylation agent comprises monochloroacetic acid or sodiummonochloroacetate.
 12. The method according to claim 1, wherein theorganic solvent comprises isopropanol, methanol, ethanol, acetone, or acombination of two or more thereof.
 13. The method according to claim 1,wherein a degree of carboxymethyl substitution per anhydrous glucoseunit in the carboxymethylated cellulose is less than 0.50.
 14. Themethod according to claim 1, wherein a degree of crystallization ofcellulose I type in the carboxymethylated cellulose is 50% or more. 15.The method according to claim 1, wherein the step of preparingmercerized cellulose is performed in a reactor, and wherein the methodfurther comprises adding an organic solvent or an aqueous solution of anorganic solvent to the reactor before, at the same time as, orimmediately after the addition of the carboxymethylation agent.